This invention relates to an ink jet printer of the asynchronous, volume displacement droplet type, and more particularly to an interchangeable print head system for such an ink jet printer.
Volume displacement ink jet printers have special ink supply requirements separate from and in addition to the requirements of "constant flow" ink jets. Since the only resupply mechanism for replacing expelled ink is capillary action, the ink supply pressure must be within the range of capillary pressure that can be generated by the print head nozzles. The preferred supply pressure is very slightly negative, perhaps the equivalent of 1 to 2 centimeters of water. This type of ink supply arrangement is discussed in U.S. Pat. Nos. 3,708,798; 3,747,120; 3,832,579; and 3,852,773.
Another important criteria for such ink jet printers arises from the low pressure requirement discussed above. Since the ink reservoir is typically connected directly to the print head to provide the low hydrostatic supply pressure, the system is vulnerable to pressure surges generated by the motion of the print head or the supply line due to inertial forces. Also, the usual arrangement of a volume displacement ink jet printer is to have a plurality of droplet ejection devices connected to the same ink supply. It thus becomes necessary to isolate the individual ejection devices from each other so that they can be independently actuated. This is done in some prior art devices by use of a pulse trap chamber which is partially filled with air to absorb pressure surges in the supply line and to isolate the individual rejection devices in the print head. See for example, U.S. Pat. No. 3,708,798. In one prior art device a passively activated valve is used to minimize the pressure surge effect by having the pressure surges themselves providing the actuating force for the valve. See, for example, U.S. Pat. No. 3,852,773. In still another prior art device, a pressure absorption mechanism is used for a single ink jet ejector so that self-generated resonances are minimized and the useful range of the device is extended to higher droplet ejection rates, see for example, U.S. Pat. No. 3,832,579.
Substantially all ink jet printing systems function best when there is no air or gas in the ink supply system or in the print head. This requirement is particularly severe for volume displacement printers since air bubbles will counteract the incompressible properties of the fluid and prevent the volume displacement mechanism from working properly. In some systems, chambers are provided for accumulating air bubbles before they reach the print head. See for example, U.S. Pat. Nos. 3,708,798 and 3,805,276. In one system, the air and gas are removed from the ink supply before it is used. See U.S. Pat. No. 3,346,869.
Another problem with many prior ink jet printer systems is that they are tuned uniquely for each print head. This makes ready replacement of defective print heads difficult and expensive. Not only must the system be retuned for the replacement ink jet print head, but the ink supply line must be flushed clear of any accumulated air which enters the system during the replacement process.